Extensive use of superalloys is made in gas turbine engines. Superalloy components in gas turbine engines operate near the limit of their properties in a severe environment, both with respect to temperature, stress and oxidation/corrosion. One of the major classes of components in a gas turbine engine are the disks upon the periphery of which is mounted a plurality of blades. The disks rotate at speeds on the order of 8,000 to 10,000 rpm and, in the turbine section of the engine, can be exposed to temperatures on the order of 1000.degree. F. to 1300.degree. F. (538.degree. C. to 704.degree. C.). During engine operation, the disks are exposed to cyclic stress conditions which can lead to failure. Every effort is made to increase the rotational speed of the engine, since by increasing engine rotational speed, increased thrust and fuel efficiency can be obtained. Counterbalancing this effort, however, is the appreciation that catastrophic disk failure cannot be tolerated. Periodic exhaustive engine inspections are made and a particular item of concern in such inspection is the possible presence of cracks in disks.
Thus, there exists a need for a disk material capable of withstanding more demanding stress conditions without catastrophic failure, and there exists a need for disk material which will not fail catastrophically, but rather will fail by slow, steady, progressive crack growth, crack growth at a rate slow enough that periodic engine inspections will uncover such cracks well before catastrophic failure can occur.
It is now appreciated that in virtually every metallic article, flaws exist; it is merely a matter of looking for the flaws on a fine enough scale--existing flaws will initiate cracks. It is believed that cracks in disks, as in most other metallic articles, originate at flaws in the material and grow in response to engine stress conditions. The object then is to have the cracks which do form grow at a slow rate, that is to say, da/dn should be minimized (where a is crack length and n is the number of stress cycles). At the same time, however, the minimization of the da/dn characteristic of the material must be accomplished without significant detriment to the other important mechanical properties of the material including creep, stress rupture, fatigue and the like.
Prior art heat treatments for disk materials have generally included what is referred to as a solution treatment step, followed by several aging steps performed at lower temperatures. In fact, however, the prior art has generally used a "solution treatment" performed below the true solution temperature of the alloy. Through the use of such a step, the gamma prime phase is not totally taken in solution; but instead, sufficient gamma prime remains to minimize grain growth. Thus, the fine grain size of the starting material is not substantially affected by the solution treatment temperature. In the prior art, it was generally believed that the maintenance of a fine grain size was essential to the achievement of desirable disk properties.
We have found, however, that a different approach to heat treatment can provide substantially improved mechanical properties. Therefore, it is an object of this invention to describe a heat treatment which can be applied to nickel base superalloy articles and which will significantly reduce the rate at which cracks grow without materially reducing other important mechanical properties.